The basic unit for transporting goods has been the truck. Being the basic unit, the truck has defined limitations on intermodal containers that can typically be transported by ships, trains and trucks. However, airplanes have generally been excluded from participation in intermodal and many other types of cargo. This is due to the limitations placed by the design and construction of cargo airplanes.
The design and construction of most civilian cargo aircraft are based on that of passenger airplanes. The basic structure is a monocoque-based fuselage which is substantially cylindrical in shape. Monocoque-based structures support the structural load of an aircraft by a unitary structural body, as opposed to heavier internal frames or trusses. The unibody construction of the monocoque-based aircraft generally lack sufficient structure to adequately or efficiently support and distribute concentrated cargo loads across the aircraft fuselage and to the wings.
Additionally, the cylindrically-shaped fuselage imposes additional restrictions on cargo size and dimensions. Thus, cargo having irregular or unusually large dimensions are generally unsuited for air transport via today's cargo aircraft. Moreover, as most cargo units are substantially rectangular in shape, loading such cargo units into a cylindrical fuselage results in a significant amount of wasted dead space.